I've been asked to fix an apple mac PowerBook G4 15'. I turn the power button on and hear the sound, and can hear the computer on and working. However, the screen remains blank. I have no idea about MACS so any help will be deeply appreciated.

Maybe the Main Logic Board, or the LCD.
The main logic board on iBooks has a cold solder joint that is ALWAYS going out, however, I don't remember if that affects Power Books.
Look hard at the screen, see if you can see ANYTHING, it may be that the inverter board has gone bad, just fritzing out the backlight.
My bet though: the MLB. Check with Apple and see if it is under warranty, or if the part is on recall. (The iBook MLBs are.)

Its under 3 weeks old. The little apple on the back does not light up which leads me to assume its got to be power related. But it's for my girlfriend, and as its under 3 weeks old...she isn't a happy bunny.

Its under 3 weeks old. The little apple on the back does not light up which leads me to assume its got to be power related. But it's for my girlfriend, and as its under 3 weeks old...she isn't a happy bunny.

If I remember right I think the apple on the back of the screen is actualy lit up by the back lighting of the screen. Hence I would go with what Omega said.

Either way it sounds like you got a defective item, it happens. My iBook works great, and I've heard nothing but good things about the powerbooks as well from some friends.

I'm waiting for the G5 powerbooks to come out before I buy one. (I know the second I buy a G4 the G5 will be released)

Actually this problem could have been as simple as a dead battery. You or Apple has no idea how long that battery has been sitting in the factory. I had an iMac that would start up, but the screen would never sync. I had a friend who worked at a mac repair shop tell me to change the battery and that fixed it. I'm not sure if it works on the laptops though.